Leeches are blood-sucking worms that live in damp places and become attached people and animals. After biting the host they feed for up to two hours, consuming between two and ten times their own weight before releasing their grip.
An anaesthetic in the saliva of leeches results in that bites that are initially painless.

Prevention

Use insecticide around socks and shoes, wearing fine weave.

Wear socks with no seam holes at the ankles, sealing trouser legs to boots with tight socks, gaiters or elastic bandages.